1. Field of the Invention
The present invention relates generally to a fluid coupling for transmitting torque from a driving member to a driven member via a fluid medium to cause the driven member to rotate. More specifically, the invention relates to a fluid coupling for transmitting rotation force from an internal combustion engine to a cooling fan.
2. Description of the Background Art
In water cooling type engines, air passing through a radiator causes heat to be radiated from cooling water. Therefore, the higher the flow rate of air passing through the radiator the better the heat radiating effect. When the vehicle is running at moderate or relatively high speeds, a sufficient amount of cooling air is forced against the front surface of the radiator. However, when the vehicular speed is low, or when the engine operates in the idling condition, the amount of cooling air is insufficient. In order that sufficient cooling air is introduced to the radiator in these cases, a cooling fan which rotates in accordance with rotation of the engine is arranged behind the radiator.
When rotation speed of the cooling fan becomes high, air resistance also becomes high, so that noise becomes great as well as the force required for rotating the fan. On the other hand, when the vehicular speed becomes high, the cooling fan does not need to draw so large an amount of air into the radiator since a large volume of air is forced against the front surface of the radiator due to air resistance to vehicle motion. For that reason, various systems have been proposed, which prevent the rotation speed of a cooling fan from increasing when the rotation speed reaches a certain value, or which cause a cooling fan to rotate only when it is required. Such systems include a fluid coupling, a fan clutch, a fan coupling and so forth.
Generally, a fluid coupling comprises a disc for receiving rotation force from an engine, and a wheel to which a cooling fan is attached. The wheel is designed to rotate in accordance with rotation of the disc via viscous resistance of silicone oil which passes through a labyrinth groove formed between circumferential surface portions of the disc and wheel. When rotation speed of the disc becomes greater than a predetermined value, the viscous resistance becomes insufficient relative to rotation speed of the disc, so that the rotation speed of the wheel does not become greater than a predetermined value.
The fan coupling has a fluid coupling which is combined with a bimetal thermostat, a sliding valve and so forth. The sliding valve is open and closed in accordance with expansion and contraction of the bimetal which expands and contracts in accordance with temperature, to adjust amount of working fluid introduced into a working chamber from a fluid storage chamber. The working fluid introduced into the working chamber is further introduced into a labyrinth groove, so that viscous resistance is produced between the disc and the wheel. In this way, the fan coupling causes the fluid coupling to operate in response to temperature of engine coolant.
In conventional fan couplings, there is a disadvantage in that, when the engine speed is rapidly increased from the engine idling condition, rotation speed of the cooling fan is rapidly increased which produces fan noise and causes power loss in the engine.
In order to overcome the aforementioned problems, Japanese Utility-Model First (unexamined) Publication (Jikkai Sho.) No. 60-58930 and (Jikkai Hei.) No. 1-131033 disclose improved fan couplings.
However, these disclosed systems have a relatively complicated valve mechanism which uses a spring or the like. Therefore, the construction thereof is not only complicated, but also many parts and high manufacturing accuracy are required. As a result, manufacturing becomes complicated, and manufacturing costs increase.